Remember in high school when you were sure that you would never need to use all that math as a grown up? Well, it turns out that that was somewhat true. Our word this week is Centroid, which is a term used in mathematics and physics to refer to the geometric center of a two-dimensional region and is the arithmetic mean or average position of all points in the shape. For us, it refers to the geographic center of a defined area, and is offered for all of our data. We use Zip4 centroid points to build our postal boundary data.
For polygons, we provide a weighted centroid because for certain shapes (such as an L-shaped polygon), a centroid calculated by arithmetic mean alone often falls outside of the polygon. A weighting algorithm is used to place the point inside the polygon. The good news? We take care of the calculations so you don’t have to. You can stick to not using any of that math you learned in high school.
A shapefile is a standard GIS data format for representing geographic features in a set of computer files. Shapefile format can includes points, lines or polygons as vectors, along with associated attribute data. For example, a polygon might represent the region of a neighborhood and an associated attribute value could be the name of the neighborhood. Shapefile format was introduced in the early ‘90s by ESRI and its definition is publicly available. A strength of the format is found in its relative simplicity. Each shapefile may only contain one type of vector geometry. Since shapefiles files lack a topographical data structure, they deliver faster drawing and edit capabilities and also require less digital storage space than more complex formats.
The actual shapefile file extension (.shp) must be used with the support of at least two additional files. An .shx file adds a positional geometry index for faster access and map drawing. The .dbf file, based on dBASE database format, is used to store the attributes. By creating large data files containing numerous data points, lines or polygons along with associated attributes, extensive maps can be created.
Shapefiles can be read, edited and their maps drawn using many standard GIS software packages.